Introduction
It has been well established for many years that hearing can be permanently impaired through prolonged exposure to high sound intensity, or by short bursts of extremely high intensity. Irreversible hearing impairment may not be detected for many years after exposure with the effect being accumulative with contributions from both prolonged exposures and intense peaks. This makes assessment and control of sound pressure levels very important for both the workplace and more general situations.
Guidelines and regulations have been improved as and when new understanding of the effects of sound exposure emerges. Acceptable levels have been generally reduced and the scope of application has broadened. Legislation to control noise in the workplace is common in many countries and has encouraged employers and employees to reappraise exposure to damaging sound pressure levels. Outside the workplace the widespread use of personal listening devices has led to widespread concerns about potential hearing damage with prolonged use.
Legislation in most parts of the world has a similar form based on generally accepted safe levels of sound. Two aspects of the regulatory limits are short-term peak levels, and dose or long-term averaged levels.
Earpiece users experience noise from both the local environment (ambient) and from the electronic sound source being delivered to the earpieces and several approaches to hearing protection are necessary. Ambient noise levels can be suppressed at source which is often encouraged as a first step. They can be suppressed at the ears though the use of appropriate well-fitting earpieces which could be in-ear-monitors (IEMs) or over-the ears headsets.
Acoustic levels delivered by the earpieces can also be limited in various ways, with source-limiting, or various limiting techniques at the earpieces. However, clear communications can be compromised if earpiece generated acoustic levels are limited to the extent that residual ambient noise is too high, either generally or for short periods. This is especially so of some simple limiters applied to earpieces at the present time.
Approaches to Hearing Protection
The most common options are based on an assessment of typical sound levels and are not specific to a particular situation. These do not always deliver as high an audio level as is safe for short periods, and so communications intelligibility can suffer significantly. Other more sophisticated options are more expensive, use more power and are relatively large in size compared with what is convenient in many applications.
Both these factors limit the availability of effective solutions to prevent potentially damaging levels of sound exposure. This invention seeks to make an effective solution more readily available.
Assessing the dose delivered by the earpieces is also usually assessed based on typical content. However the exposure from earpieces is highly variable during each day, between different user's environments and from day to day.
These, can help establish appropriate measures to be implemented, but do not deal with any specific user and particular working occasion. In setting a limit for the earpiece dose based on such assessments, the limited level may be insufficient for clear communications, which in some situations can lead to compromised safety. Such solutions fail to make use of the full range of safe dose levels as defined by the legislation.
Protection in the Source Device: The source device such as a radio handset or MP3 player can limit the signal delivered to the earpiece. The most common form of limitation is of voltage, such as a peak level of voltage output to the earpiece. This could be related to the device's output stage capability or to the setting of a volume control. Such schemes rely on 4 basic assumptions: The relationship between peak level and the averaged energy over time, the sensitivity of a particular earpiece, the period of time that the device is in use for, and the frequency content of the audio signal (mid-to high frequencies being more damaging than low frequencies). Mistakes in any of these can result in hearing damage.
Nevertheless, assessments have been made and several radio handsets do incorporate such simple measures. Some can detect different earpieces being connected and make some adjustment for these. However, a test tone at a certain peak level has many times more potential to damage hearing than occasional spoken words at the same peak level. This leads to loss of communication clarity in some adverse ambient noise environments.
Protection in the Earpiece or Headset: Some form of limiting device can be applied either between the source and earpiece, or incorporated in the earpiece itself. There are several types.
The most basic is a fixed attenuator, commonly a fixed series resistance in the earpiece lead. Such a scheme relies on knowledge of the peak drive capabilities of the source device, the average energy over time and frequencies of the programme material and the overall period of exposure. Such an approach often results in either inadequate clarity of communication or inadequate hearing protection.
Simple hard limiters can be fitted, such as cross-coupled diodes. These limit the peak voltage getting to the earpiece, but suffer from all of the problems associated with the above as well as significant distortion of peak signals and inadequacy in terms of limiting acoustic energy in the ear to safe levels.
There are some self-powered level control schemes implemented using voltage-controlled gain devices such as MOSFETs or JFETs. One of the earliest was been designed by the BBC and is available commercially. The level of attenuation is determined by a simple short-term average of the rectified input signal. Acting as a soft limiter, such schemes allow slightly higher acoustic levels for very short periods of time, but then the attenuation brings the level down to a lower level. They do not base the controlled level on the output and are consequently not accurate. The operating limit still has to be based on an assessment of programme content as it is not able to monitor actual dose or respond specifically to accumulated dose. However, they are much better solutions than fixed attenuators. Some variants are not able to work at some of the lower signal levels used in IEMs, but others incorporate transformers which allow the necessary high control voltages to be generated. Some form of individual calibration is necessary to remove the large uncertainties in the MOSFET or JFET control characteristic.
Similar devices and schemes have been incorporated in some headsets to limit peak signal levels to prescribed levels. They have no effect on the programme content until excessive levels occur and are much more effective than the hard limiters described above. However, they do not deal with hearing dose exposure.
Protection Based on Dose Measurement: There are solutions that monitor dose continuously and either provides an indication of dose level or take action to limit the level. Because of the computation necessary to determine dose, these are generally based on sophisticated processing in digital signal processors (DSPs) and hence are relatively expensive in comparison with the cost of many hearing devices, need more power than is easily available in most circumstances, and are bulky. Hence they are not optimum solutions for the majority of earpiece users.
One approach is to incorporate the attenuation function within the DSP. However this necessitates additional power to drive the hearing device. Another approach is for the DSP to control an external attenuator such as a JFET or MOSFET device. This invention could be applicable to such an approach.
Methods of Attenuation
Resistive Attenuator: FIG. 3 shows a fixed attenuator configuration with resistance R1 and the impedance of the earpiece defining the level of attenuation. This reduces the level of the input signal (6A) to produce the drive signal (6B) for the earpiece (1). The common connection (10) is ground in most applications.
Any amount of attenuation is possible through selection of an appropriate value for R1. However this offers no inherent protection for dose or for peak signal levels.
Earpieces do not have constant impedance over their frequency range and are generally designed to work from low source impedances. Hence there is likely to be an undesirable effect on frequency response and audio quality, especially with high levels of attenuation. Such effects can be reduced by applying a resistive shunt element to the earpiece as shown in FIG. 4. For a given level of attenuation, R1 will have a lower value; the impedance presented to both the signal source and to the earpiece will be correspondingly lower. For most applications this will be perfectly acceptable.
Basic Shunt Voltage Controlled Attenuator: Prior art has many forms of a JFET (junction-field-effect-transistor) or MOSFET (metal oxide field effect transistor) in use as a voltage controlled attenuator. A basic configuration is shown in FIG. 5 by way of introduction to the present invention.
Q1 is a MOSFET in this representation, but could be another device such as a JFET or a more complex arrangement embedded within an integrated circuit. One example of this could be a chopping network followed by a low-pass filter.
Q1 has a control terminal (gate-G) and 2 terminals which can act as a variable resistance (RDS) between the drain-D and source-S. In this configuration, Q1's RDS effectively presents a shunt impedance to the earpiece (1). RDS is dependent on the control voltage (5) which thus determines the level of attenuation. When used with AC (alternating current) signals, the control voltage is with respect to ground (10).
MOSFET devices can have a minimum RDS resistance of only a few milliohms whilst JFETs have minimum RDS resistance of a few ohms. Both have extremely high maximum RDS, effectively open-circuit. The range of control voltage for most MOSFETs is between 0V (off or open-circuit RDS) to the gate threshold voltage which could be <1V for logic level devices or several volts for others. For JFETs, the range of control voltages is between 0V (on, minimum RDS) to the gate cut-off voltage, which is likely to be a few volts negative for N-type devices, those with the lowest RDS. J108 for example can have RDS minimum of less than 8 ohms and a gate cut-off voltage anywhere between −3 and −10V, which illustrates the manufacturing variability of such devices.
With only modest values of R1, considerable attenuation is possible with a MOSFET, but relatively modest attenuation with a JFET due to its limited RDS range. However MOSFETs have an integral diode to the substrate that shunts one phase of large AC signals. A back-to-back configuration can be employed to overcome this, but at the expense of additional complexity.
Improving Linearity: JFETs and MOSFETs are typically symmetric when used as a controlled resistance for small signal levels. This implies the control voltage reference point is midway between the voltages of source (S) and drain (D) rather than ground (10).
One well known way of achieving this is illustrated in FIG. 6. A proportion (50%) of the output AC signal is fed to the gate of Q1. If the source impedance of control voltage (5) is low proportion this is achieved with R3=R4. This necessitates the control voltage (5) to be twice that in FIG. 5 for a given level of attenuation. A useful reference to the use of FETs as low-distortion attenuators is Siliconix AN104 (10 Mar. 1997).
It is also well known that a capacitance can be included in series with R3 to isolate any DC between the signal and control sections. The time-constant formed by R3 and the capacitor must be large enough to pass all signal frequencies and yet small enough to block changes in control voltage affecting the signal. For audio applications and where control changes are slow, this is not a problem.
However, even with the above linearization scheme, JFETs and MOSFETs tend to introduce distortion when RDS resistance is high. According to the AN104 reference, this occurs with RDS being over ten times RDS minimum and this can be readily be observed using distortion measuring equipment. With the device set off (extremely high RDS) so that it has no effect on the signal and there is minimum attenuation, then distortion is introduced as small amounts of attenuation are introduced; with greater attenuation the distortion falls away again. If RDS was restricted to a range of 1:10 to minimise distortion, this would significantly restrict available attenuation.
It is possible to configure the JFET or MOSFET as the series element and replacing R1 of FIG. 6. In this situation distortion would be introduced at higher levels of attenuation which may be more acceptable in some applications.
Generation of Gate Control Voltages
Hearing dose management necessitates long-term assessment of hearing dose, most easily achieved in some form of computation device such as a DSP or microcontroller. Logic level MOSFETs can easily be supplied with suitable control voltages within the supply and operating voltages of such devices. However high audio voltage levels are best managed with JFETs that necessitate much higher and negative gate control voltages. Hence some means of generating control voltages with a typical range of 0V to −10V is required. This is usually a simple task, but becomes a significant challenge when the lowest possible power consumption is required. Some general approaches are reviewed here but a novel approach is presented within the details of this invention:
Regulated Supply and High Voltage Amplifier: A high voltage can be created using a switched regulated power supply, either an inductive inverter or a capacitive charge pump; this could be fed from a low supply voltage such as a 3V battery. A low voltage control signal from a microcontroller can then be amplified through a suitable amplification stage to deliver the 0=>−10V.
Most power supply regulation techniques rely on there being a reasonable load on the power supply; the output is actively increased if it is too low, but passively decreased if it is too high through power being drawn by the load. This and the use of relatively high-voltage amplifiers both tend to compromise micro-power applications.
Unregulated Supply and Low Voltage Amplifier: It is possible to use an unregulated power supply and a low-voltage amplifier in conjunction with an external device such as a MOSFET; certain configurations can achieve stable gate drive levels. FIG. 9 shows an inverting voltage conversion stage that can generate gate control voltages from a low voltage control input without needing a regulated supply or high-voltage amplifier.
Apart from the added complexity, such configurations have a limited negative slew rate capability. MOSFET (Q10) is actively involved for positive ramp whilst the resistance (R18) to the negative supply (−12V UNREGULATED) is only passively pulling the voltage down. These would have high values to minimise power consumption and yet low values to rapidly change the voltage on an capacitors associated with the JFET gate control.
A shunt JFET would be able to rapidly attenuate sudden incoming excessive signals; however it would only be able to slowly reduce the attenuation. If a series JFET were used (as in the position of R1 in FIG. 4) then it would be unable to rapidly increase attenuation. For some applications these limitations may be acceptable.
Supply+Pulse-Width Modulation: By using pulse-width modulation in conjunction with a regulated power supply it is possible to transform a logic level output to any voltage within the power supply range with very low power consumption apart from the need to keep the microcontroller or similar device running. This has the advantage of relative simplicity when compared with FIG. 9. The supply in this case could be a regulated 0V to −10V and the logic level could be 0V to +2.5V used by a microcontroller.
Direct Generation of Control Voltage: Rather than have separate power supply to generate the negative voltage and then some form of amplifier to use it to generate the gate control voltage, it is possible to use some form of inverter to generate the voltage directly. Transformers, inductors or capacitors as charge-pumps can all be used. Where control speed is important, these schemes will have a similar problem with rising amplitude ramps being faster than falling amplitude in low-power applications.
Fail Safe Operation: Any system providing protection that depends on power should have some scheme that ensures safe operation in the event of power failure. A shunt MOSFET will normally be enhancement mode and so go to the off-state of high RDS in the event of power failure and loss of the gate control voltage; this would provide minimum attenuation and least protection. A shunt JFET will go the minimum RDS condition with loss of gate control voltage and hence provide some protection on power failure. A series MOSFET would go to higher attenuation on power failure whilst a series JFET would go to minimum attenuation.
Reducing Device Variability
It is well known that MOSFET and JFET characteristics vary significantly from device to device, and to some extent even those in a particular manufacturing batch. It is also well known that these characteristics are highly dependent on temperature. This makes defining a particular control voltage to attain a particular level of attenuation very difficult, without resort to elaborate calibration and temperature compensation schemes. Prior art U.S. Pat. No. 7,750,738 used a configuration of 2 MOSFET devices to overcome all of these issues for integrated circuit designs. As dual MOSFET devices are available in discrete form (eg Fairchild Semiconductors FDS8984, being one of many similar devices), it is possible to make similar arrangements using 2 fairly well matched devices, in terms of both process and having the same temperature.
As previously mentioned, some forms of limiter employing JFETs or MOSFETs include a means of adjustment such as a variable potentiometer so as to compensate for at least variability in control characteristic. It is also possible to automate aspects of this through laser trimming of resistor networks.
Other than this, some means of calibration is necessary. A microcontroller or DSP device allows calibration parameters to be held indefinitely and can be included in simple automated schemes within the contest of manufacturing test.